The present invention relates to electronic compasses. More particularly, the present invention relates to compensation of electronic compasses for magnetic errors.
Electronic compasses are well known in the art. Such devices typically have used magnetic flux gate or other sensors to measure magnetic fields and to determine orientation with respect to the Earth""s magnetic field. As with needle- or card-based compasses, however, when an electronic compass is used in an environment of ferrous metals and associated perturbing fields, the fields sensed by the magnetometer sensors are perturbed, leading to erroneous readings of the Earth""s magnetic field and the compass magnetic azimuth.
To obtain correct readings, it is necessary to compensate for these magnetic perturbations. Compensating needle- or card-based compasses requires the use of bar magnets and/or soft iron masses to physically neutralize and cancel perturbing magnetic fields. These magnets and soft iron masses must be positioned carefully about the compass so as to cancel preexisting perturbations and reduce the deviation errors to, for example, 3-5xc2x0. Even after this reduction, however, residual deviation errors must be plotted against true azimuth so that the user can correct the azimuth value.
Electronic compasses, which use microprocessors to process the data received from the magnetometer sensors, can be compensated using numerical methods. One particular example of compensation using numerical methods is the classical five-term compensation formula for a level compass. In this formula, the deviation of the aximuth as measured by the compass from the true azimuth is expressed in degrees as a function of the true magnetic azimuth xcex8 as follows:
Deviation=A+Bxc2x7sin(xcex8)+Cxc2x7cos(xcex8)+Dxc2x7sin(2xcex8)+Exc2x7cos(2xcex8)
where A, B, C, D, and E are coefficients whose values are determined through some calibration procedure. This compensation technique exhibits certain limitations. For example, the above formula is only an approximate expression, valid only for small deviations and small values of A, B, C, D, and E. Thus, this technique is only used after physical compensation to reduce deviation errors, e.g., through the addition of magnets and/or soft iron masses. In addition, the formula is valid only for a level compass and is therefore poorly suited for use in environments in which the orientation of the compass may vary widely within three-dimensional space, such as heeled ships. Changes in latitude can also affect the quality of compensation. While this compensation technique is reasonable for use also with aircraft flux gate compasses, it is only approximate for common pendulous flux gate compasses, as the coefficients are dependent on tilt attitude and magnetic latitude.
Certain conventional compensation techniques have been implemented, but many are limited to two-axis compasses and do not produce adequate results for a variety of arbitrary orientations of the compass in three-dimensional space. Some conventional compensation techniques have been applied to three-axis compasses. Such conventional techniques, however, fail to adequately compensate for certain types of errors because they rely on symmetric coefficient matrices.
The present invention addresses these and other problems by using a three-axis physical model to numerically compensate for errors in measured magnetic field values in an electronic compass. This model is based on physical principles rather than approximations, and thus produces more accurate compensation. Furthermore, the model uses a linear algebra approach that facilitates application of the compensation and determination of the parameters needed for compensation. By using a three-axis physical model, the present invention compensates an electronic compass over all orientations and is not limited to use in relatively level orientations.
According to one embodiment, the present invention is directed to a method of compensating an electronic compass to obtain accurate azimuth data despite the presence of perturbing magnetic effects. For each of a plurality of combinations of orientations and azimuths, a measured magnetic field vector HMEAS representing magnetic field strength in three axes and a measured gravity vector GMEAS representing gravitational field strength in three axes are obtained. These vector values are used to calculate a matrix compensation coefficient LE and a vector compensation coefficient HPE using a system of equations. The matrix compensation coefficient LE and the vector compensation coefficient HPE are then used to correct subsequently measured magnetometer output data to obtain the accurate Earth""s magnetic field data.
Additional embodiments are directed to electronic compass arrangements and microprocessor-readable media for performing these methods. The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description that follow more particularly exemplify these embodiments.